Conventionally, as general memories used for information processors such as a computer and a communication device, volatile memories such as a DRAM (Dynamic Random Access Memory) and an SRAM (Static RAM) are used. The volatile memories have to be refreshed by always supplying current to hold stored information. When the power source is turned off, all of information is lost, so that a nonvolatile memory as means for recording information has to be provided in addition to the volatile memory. For example, a flash EEPROM, a magnetic hard disk drive, or the like is used.
In the nonvolatile memories, as the speed of information processing increases, increase in speed of an access is becoming an important subject. Further, as a portable information device is being rapidly spread and the performance is becoming higher, information device development aiming at so-called ubiquitous computing which means that information processing can be performed everywhere at any time is rapidly being progressed. Development of a nonvolatile memory adapted for higher-speed processing as a key device of such information device development is in strong demand.
As a technique effective to increase the speed of the nonvolatile memory, a magnetic random access memory (MRAM) in which magnetic memory elements each for storing information in accordance with the magnetization direction along the axis of easy magnetization of a ferromagnetic layer are arranged in a matrix is known. The MRAM stores information by using a combination of the magnetization directions in two ferromagnetic members. On the other hand, stored information is read by detecting a resistance change (that is, a change in current or voltage) which occurs between the case where the magnetization direction is parallel to a reference direction and the case where the magnetization direction isantiparallel to the reference direction. Since the MRAM operates with the principle, it is important that the resistance change ratio is as high as possible to perform stable writing and reading in the MRAM.
The MRAM currently used in practice utilizes the giant magnetoresistive (GMR) effect. The GMR effect is a phenomenon such that when two magnetic layers are disposed so that their axes of easy magnetization are parallel to each other, in the case where the magnetization directions of the layers are parallel to the axis of easy magnetization, the resistance value becomes the minimum. In the case where the magnetization directions of the layers antiparallel to the axis of easy magnetization, the resistance value becomes the maximum. An MRAM using a GMR device capable of obtaining such a GMR effect (hereinbelow, described as GMR-MRAM) is disclosed in, for example, U.S. Pat. No. 5,343,422.
The GMR-MRAM has a coercive force difference type (pseudo spin valve type) and an exchange bias type (spin valve type). In the MRAM of the coercive force difference type, the GMR device has two ferromagnetic layers and a nonmagnetic layer sandwiched between the two ferromagnetic layers and, by using the difference between the coercive forces of the two ferromagnetic layers, information is written/read. In the case where the GMR device has a configuration of, for example, “nickel iron alloy (NiFe)/copper(Cu)/cobalt(Co)”, the resistance change rate is a small value of about 6 to 8%. On the other hand, the MRAM of the exchange bias type, the GMR device has a pinned layer whose magnetization direction is pinned by antiferromagnetic coupling to an antiferromagnetic layer, a free layer whose magnetization direction changes according to an external magnetic field, and a nonmagnetic layer sandwiched between the pinned layer and the free layer. By using the difference between the magnetization direction of the pinned layer and the magnetization direction of the free layer, information is written/read. For example, the resistance change rate of the GMR device having a configuration of “platinum manganese (PtMn)/cobalt iron (CoFe)/copper (Cu)/CoFe” is about 10% which is higher than that of the coercive force difference type. However, it is insufficient to achieve improvement in storing speed and access speed.
To solve the problems, an MRAM having a TMR device using tunneling magnetoresistive effect (TMR) (hereinbelow, written as “TMR-MRAM”) is proposed. The TMR effect is an effect such that the tunnel current passing through an insulating layer changes in accordance with relative angles of the magnetization directions of two ferromagnetic layers sandwiching a very-thin insulating layer (tunnel barrier layer). When the magnetization directions of the two ferromagnetic layers are parallel to each other, the resistance value becomes the minimum. When the magnetization directions antiparallel to each other, the resistance value becomes the maximum. In the TMR-MRAM, when the TMR device has a configuration of, for example, “CoFe/aluminum oxide/CoFe”, the resistance change ratio is high as 40% and the resistance value is also large. Consequently, the TMR-MRAM can be easily matched with a semiconductor device such as an MOSFET. Therefore, the TMR-MRAM can easily obtain a higher output as compared with the GMR-MRAM, and improvement in storage capacity and access speed is expected. In the TMR-MRAM, a method of storing information by changing the magnetization direction of a magnetic film of the TMR device by a current magnetic field generated by passing current to a conductor is known. As a method of reading stored information, a method of passing current in a direction perpendicular to a tunnel barrier layer and detecting a resistance change in the TMR device is known. Techniques on the TMR-MRAM disclosed in U.S. Pat. No. 5,629,922 and Japanese Patent Laid-open No. Hei 9-91949 and the like are known.
As described above, an MRAM using the TMR effect can achieve an output higher than that of the MRAM using the GMR effect. However, since an output voltage of even the MRAM using the TMR device achieving a resistance change rate of about 40% is tens mV, it is insufficient to realize a magnetic memory device of higher packing density.
FIG. 48 is a plan view showing the configuration in a magnetic memory device using the conventional TMR effect. FIG. 49 shows a sectional configuration of the main part of the conventional magnetic memory device corresponding to FIG. 48. A write bit line 105 is orthogonal to a read word line 112 and a write word line 106 extending in parallel with each other, and a TMR device 120 constructed by a first magnetic layer 102, a tunnel burrier layer 103, and a second magnetic layer 104 is disposed in an area sandwiched in the Z direction of the orthogonal portion. In such an MRAM of the type in which the write bit line 105 and the write word line 106 are orthogonal to each other, the magnetization direction of the second magnetic layer 104 functioning as a free layer cannot be sufficiently maintained as a whole, and it is difficult to perform sufficiently stable writing.
In the MRAM using the TMR effect, information is stored in each of memory cells by changing the magnetization direction of the magnetic film by an induction field by current flowing in conductors arranged orthogonal to each other, that is, current magnetic field. Since the current magnetic field is an open magnetic field (which is not magnetically confined in a specific area), the efficiency is low and an adverse influence on neighboring memory cells is also concerned.
Further, in the case of increasing integration of memory cells to achieve higher packing density of a magnetic memory device, it is essential to make a TMR device finer. However, it is feared that demagnetizing field increases as the aspect ratio (thickness/width in the direction in the stack layer plane) of each of the magnetic layers in the TMR device becomes higher, the magnetic field intensity to change the magnetization direction of the free layer increases, and larger write current is necessary.